Two-Wire Power And Communications For Irrigation Systems

ABSTRACT

A two-wire power and communication system is provided, having a decoder that draws a constant amount of current for communication signals despite changes in the voltage on the power and communication wires. In one example, decoders have a constant current sink circuit that includes a shunt regulator that controls a field effect transistor.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/763,893 filed Apr. 20, 2010 entitled Two-Wire Power AndCommunications For Irrigation Systems, issuing on Sep. 25, 2012 as U.S.Pat. No. 8,274,171, which claims priority to U.S. ProvisionalApplication Ser. No. 61/171,027 filed Apr. 20, 2009 entitled ConstantCurrent ACK Pulse For Two-Wire Decoder, and to U.S. ProvisionalApplication Ser. No. 61/171,015 filed Apr. 20, 2009 entitled SurgeProtection For Two-Wire Decoder, the contents of all of which areincorporated in their entireties herein.

BACKGROUND OF THE INVENTION

Large commercial irrigation systems such as those used on golf coursesor croplands use sprinklers, sensors or other components that arenormally powered from 24 V AC power lines that can be several miles longand can serve many hundreds of components. Various systems have beenproposed for powering and controlling the components of such a systemwith just two wires. For example, U.S. Pat. No. 3,521,130 to Davis etal., U.S. Pat. No. 3,723,827 to Griswold et al., and U.S. Pat. No.4,241,375 to Ruggles disclose systems in which sprinkler valves along acable are turned on in sequence by momentarily interrupting the power ortransmitting an advance signal from time to time.

A problem with this approach is that it does not allow the operator tofreely turn on or off any selected sprinkler or set of sprinklers atdifferent times. This problem is typically resolved by providingseparate controllers in the field to operate groups of sprinklers inaccordance with a program stored in them, or transmitted to them byradio or other means. Alternatively, it has been proposed, as forexample in U.S. Pat. No. 3,578,245 to Brock, to operate individualsprinkler sets from a central location by superimposing afrequency-modulated signal or DC pulses onto the 24 V AC power line.

All of these approaches are expensive. For example, a system withhundreds of sprinklers requires miles of expensive, heavy wiring toaccommodate the current drawn by a large number of valves that may bewatering simultaneously. Also, heavy use of D.C. current may causeelectrolysis issues with electrical components.

One alternative to these traditional irrigation systems are two-wirepower and communications systems, such as the system shown in U.S.application No. 7,358,626, the contents of which are incorporated byreference. In such systems, two wires supply both A.C. power and digitalcontrol communications from a controller to a plurality of decoders.

While these A.C. power and digital communication systems generally workwell, they also have several disadvantages. First, decoder circuitrythat listens for communications are sensitive to power surges bylightning. Hence, expensive, external surge devices with ground rodsmust be installed at short intervals along the power and communicationwires.

Second, some two wire power and communication systems use bursts ofcurrent by valve or sensor decoders to acknowledge or otherwisecommunicate with a central controller. These decoders relied on fixedresistor circuits to switch to a circuit path with a resistor betweenthe common current and ground on the decoder. Since these resistorcircuits resist at a fixed value, any reduction in the voltage causes aproportional reduction in the current pulse (e.g., due to Ohm's Law).Therefore if the current burst of the acknowledgement pulse is reducedsufficiently, the gateway 16 will not recognize it and therefore willfalsely determine that the decoder did not receive a command. Asdecoders are installed further away from a gateway or centralcontroller, the resistance of the power and communication wires becomessignificant and reduces the voltage available across the decoder.Additionally, large numbers of decoders may also further reduce voltageand increase resistance on the power and communication wires. Hence,these systems effectively become limited in size and in the number ofdecoders.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a two-wire power andcommunication system is provided, having a decoder that draws a constantamount of current for communication signals despite changes in thevoltage on the power and communication wires. In one example, decodershave a constant current sink circuit that includes a shunt regulatorthat controls a field effect transistor.

In another aspect of the present invention, the number of external surgeprotection devices are reduced or eliminated by bypassing the sensitivedecoder components of each decoder with surge protection circuits. Eachsurge protection circuit connects to a path in communication with thetwo power and communications wires, as well as paths connecting to eachvalve or sensor control wire (e.g., power wires connecting to a solenoidvalve). In this respect, fewer external surge protection devices arerequired, reducing the cost of installation while decreasingsurge-related damage (e.g., from lightning).

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of which embodiments ofthe invention are capable of will be apparent and elucidated from thefollowing description of embodiments of the present invention, referencebeing made to the accompanying drawings, in which

FIG. 1 illustrates a two-wire power and communication irrigation systemaccording to the present invention;

FIGS. 2 a-2 b illustrates example communication protocols according tothe present invention;

FIG. 3 illustrates a circuit diagram of a constant current circuitaccording to the present invention;

FIG. 4 illustrates a partial view of an external surge protector deviceand a decoder according to the present invention; and,

FIG. 5 illustrates a circuit diagram of a decoder with internal surgeprotection according to the present invention.

DESCRIPTION OF EMBODIMENTS

Specific embodiments of the invention will now be described withreference to the accompanying drawings. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art. Theterminology used in the detailed description of the embodimentsillustrated in the accompanying drawings is not intended to be limitingof the invention. In the drawings, like numbers refer to like elements.

FIG. 1 illustrates an overview of a two-wire power and communicationsystem 10 according to the present invention. A controller 14, such as aPC or stand-alone irrigation controller, is used to program irrigationschedules and to monitor sensor data from the system 10.

A gateway 16 (i.e., a communication interface) is in communication withthe controller 14 via communication connection 35 and transmitsacknowledgments or other device information (e.g., from sensors 37, 39and 43; watering station decoders 22; or sensor decoders 24) to thecontroller 14. Additionally, the gateway 16 transmits communication datasuch as commands (e.g., open or close a valve) to specific devicedecoders such as watering station decoders 22 and sensor decoders 24

In one example embodiment, the gateway 16 contains a motherboard 17 anda pair of daughterboards 19 a and 19 b that receive power from a powersource 18. Each of the daughterboards 19 a, 19 b selectively appliespotentials to wires A and B of their respective cables 20 (e.g., 1. +40V on A with respect to B; 2. +40 V on B with respect to A; or 3. anequal potential on both A and B). The daughterboards 19 a, 19 b are alsoequipped to detect current drawn by the decoders of the system, and toreport that information to the control unit 14 through the motherboard17. Device decoders such as watering station decoders 22 and sensordecoders 24 are connected in parallel to the wires A and B, and arearranged to operate the system components (e.g. water valves 26 orsensors 28) connected to them.

Note that while wires A and B (i.e., a wire pair) are described as two,single wires, it should be understood that these wires can be made up ofmultiple wires connected together in series. In other words, wires A andB refer to two different conductive, electrical paths.

An example protocol for the operation of the system of this invention isillustrated in FIGS. 2 a-d. In this example, the daughterboards 19 a, bimpress a square wave 53 alternating between +40 V (A positive withrespect to B) and 40 V (B positive with respect to A) across theirrespective outputs A and B at a 60 Hz rate. This provides a square-wavepower supply (FIG. 2 a) to all the decoders 22, 24 along the cable 20.As pointed out below, the decoders 26, 28 can use power of eitherpolarity. Because the time of the circuit at one polarity is generallyequal to its time at the other polarity, electrolysis problems areminimized.

If it is now desired to actuate a specific sprinkler or sensor, thecommand pulse train 52 shown in FIG. 2 b is transmitted. The commandtrain begins with a no-power segment 54 in which the wires A and B areboth grounded for 1/120 second. This is followed, in the preferredembodiment, by eight pulses 56 separated by similar no-power segments ordelimiters 54. The pulses 56 may be either +40 V (signifying a “1”) or−40 V (signifying a “0”). Taken together, the pulses 56 define thedesired runtime (in minutes) of the device now to be selected.

The next twenty pulses 58, again separated by no-power delimiters 54,define the address of the desired device 26 or 28. Next, the nature ofthe desired command is specified by the four pulses 60. The commandpulse train 52 illustrated in FIG. 2 b may, for example, convey thecommand “Turn Station 3 of decoder 2873 on for 25 minutes”. Uponcompletion of the command pulse train, the microprocessor returnscontrol of the wires A and B to the power relays. The output of gateway16 thus resumes the square-wave format of FIG. 2 a.

If a selected decoder 26 has received and understood the command (seeFIG. 2 c), it sends an acknowledgment signal by momentarily drawing ahigh current burst 62 during the +40 V portion of the first square wave64 following the command pulse train. This is detected by a currentsensor of the gateway 16 and constitutes an acknowledgement that thedecoder has received its instruction. If no current is detected duringthe first square wave 64, a control failure is indicated, and themicroprocessor may transmit an alarm to the control device 14.

If the addressed device was a sensor decoder 28 (see FIG. 2 d), thechosen decoder responds with current bursts 66 during the eight (in thepreferred embodiment) square waves 68 following the command train. Ineach of these square waves, a current burst 70 during the +40 V portiontransmits a “1” to the gateway 16, while a current burst 71 during the−40 V portion transmits a “0”. As in the case of a station decoder 26,the lack of any current burst during a square wave 68 indicates a systemfailure and may trigger an alarm. Additional operational details of thissystem can generally be found in U.S. application No. 7,358,626, thecontents of which are incorporated by reference.

In prior art two wire power and communication systems, the previouslydescribed current bursts or current pulses are generated by using atransistor to switch to a circuit path with a resistor between thecommon current and ground on the decoder. In this respect, this resistorcircuit creates current bursts.

These prior art decoders work relatively well when wired close to thegateway 16. However, when these decoders are installed further away fromthe gateway 16, the resistance of the communication wire A and B becomessignificant and reduces the voltage available across the decoder (e.g.,22 or 24). Since this prior art resistor circuit is a fixed resistancevalue, any reduction in the voltage causes a proportional reduction inthe current pulse (e.g., due to Ohm's Law). Therefore if the currentburst of the acknowledgement pulse is reduced sufficiently, the gateway16 will not recognize it and therefore will falsely determine that thedecoder did not receive a command.

In one embodiment of the present invention seen in FIG. 3, the decoders(e.g., 22 and 24) include a current drawing circuit 100. This circuit100 can draw a specific amount of current that is mostly unaffected bythe voltage within a relatively large voltage range. In this respect,the system 10 can include more decoders (e.g., 22 or 24) and can spacethose decoders at greater distances from the gateway 16.

Generally, the circuit 100 is a constant current sink, which allows thecircuit to draw a predetermined current that is generally independent ofthe voltage applied to it. Of course, it should be understood thatextreme voltages, such as near-zero voltages will likely not allow forsuch a constant current draw.

Turning to the details of the circuit 100, power (e.g., 40 volt A.C.power) is provided on communications lines A and B, which connect eitherdirectly or indirectly to a bridge rectifier 102. The bridge rectifier102 converts the A.C. power to D.C. power (e.g., 40 volt A.C. power to40 volt D.C. power).

A 5 volt regulator 104 is connected to the bridge rectifier 102,stepping down the power to 5 volts to power the decoder circuits 106.The decoder circuits 106 can determine which water valves 26 to turn onor can receive sensor readings from sensor 28 via communication wires108. Additionally, this 5 volts supplied to the decoder circuit can beused to power the valves 26 (e.g., solenoid valves) or power the sensors28.

When the decoder circuit 106 determines that an acknowledgement signal(i.e., a predetermined current draw or pulse) should be activated, thedecoder circuit 106 applies voltage (e.g., 5 volts) via its feedbackcontrol mechanism 119 for the desired length of the current draw. Thefeedback control mechanism 119 provides power to a resistor 120 andcapacitor 118 to filter the voltage.

Next, this power from the feedback control mechanism 119 passes to afield effect transistor or FET 112, acting as a reference voltage toopen a gate of the FET 112 and thereby allowing power from the bridgerectifier 102 (via resistor 110) to pass through. This power from thebridge rectifier 112 is then supplied to the R Sense resistor 114 (e.g.,5.8 ohms) and then to a shunt regulator 116 (e.g., LMV431 shuntregulator). The shunt regulator 116, then supplies an appropriatevoltage to the gate of the FET 112, causing it to conduct the desiredamount of current.

In this respect, a type of feedback loop is created, such that voltageacross the R sense resistor 114 causes the FET 112 to be adjusted todraw an appropriate amount of current. Therefore, as the voltage alongthe wires A and B becomes lower due to increased resistance, the shuntregulator 116 adjusts the FET 112 to decrease resistance, causing thesame amount of current to be drawn. Hence, a standard amount of currentcan be drawn despite unexpectedly low voltages being supplied via wiresA and B.

Typically, two-wire power and communication systems typically includedecoders that are installed in valve boxes below ground level or burieddirectly in the ground. Further, these decoders typically have long runsof wire that connect the decoders with the valves or sensors. Thisarrangement causes the decoders to be especially susceptible toelectrical surges from lightning.

To combat lightning surges, system manufacturers generally recommendthat external surge devices with ground rods be installed at shortintervals along the two communications and power wires. Additionally,external surge devices with ground rods need to be installed on thedecoder outputs to protect them from lightning surges. However, thesegrounding rods are typically expensive to purchase and install, makingideal grounding cost prohibitive.

FIG. 4 illustrates an example grounding arrangement according to thepresent invention in which surge protection is located within eachdecoder 22. In this respect, a system 10 may include fewer or even noexternal surge protectors and ground rods connected to the decoderoutput wires 25 (i.e., the output wires connecting the output of thedecoder 22 with valves 26).

FIG. 5 illustrates an example schematic of a decoder 22 connected to twovalves 26. Communication lines A and B are connected to the decoder 22and conductive paths for each continue within the decoder 22, connectingto the decoder components 152 (e.g., including the previously describedcurrent drawing circuit 100).

Additionally, the decoder 22 includes a surge circuit 150 for each wire25 connecting to the valves 26. In the present example, two valves 26are present with two wires each. Hence four surge circuits 150 areincluded within the decoder 22. Each surge circuit 150 connects to afirst location between the wires A or B and the decoder components 152;and to a second location on one of the wires 25 leading to the valve 26.In this respect, surge circuits 150 allow large electrical surges tobypass the decoder components that are typically sensitive to surges andexpensive to replace.

In one example, the surge circuits 150 are composed of gas dischargetubes. These gas discharge tubes can be regarded as a very fast switchhaving conductance properties that change very rapidly, when breakdownoccurs, from open-circuit to quasi-short circuit.

In another example, MOVs and TVS diodes can also be used as surgecircuits 150. Additional example surge mechanisms can be found in U.S.Pat. Nos. 5936824; 5909349; 5808850; 5500782; 5122921; 4851946; 4009422;and 3522570, the contents of which are hereby incorporated by reference.

Although the invention has been described in terms of particularembodiments and applications, one of ordinary skill in the art, in lightof this teaching, can generate additional embodiments and modificationswithout departing from the spirit of or exceeding the scope of theclaimed invention. Accordingly, it is to be understood that the drawingsand descriptions herein are proffered by way of example to facilitatecomprehension of the invention and should not be construed to limit thescope thereof.

1. An irrigation system comprising: an irrigation controller forexecuting irrigation schedules; an irrigation interface in communicationwith said irrigation controller; said irrigation interface connected toa wire pair and configured to provide a potential across said wire pair;said irrigation interface further configured to manipulate saidpotential to transmit communication data; and, a decoder connected tosaid wire pair to receive power and communication signals from saidirrigation interface; said decoder being connectable to controloperation of an irrigation valve; and said decoder having acommunication circuit for drawing a predetermined current on said wirepair independently of a voltage on said wire pair.